This invention relates to a process for the preparation of photographic emulsion and, in particular, to a process for precipitating silver halide particles using a pulsed flow, balanced double jet precipitation technique.
The preparation of silver halide emulsion used in the manufacture of photosensitive film must be carried out under carefully controlled conditions. Typical of the factors which must be controlled are the rate of addition of reactants, the silver ion concentration (pAg), the hydrogen ion concentration (pH), the halide salt concentration, the relative mixing uniformity of reactants added from two separate silver and halide sources, the temperature at which silver halide precipitation is to occur and the duration of the precipitation.
It is known that in the preparation of monodisperse narrow particle size silver halide emulsions by balanced double jet precipitation rapid dilution of the reactants in a reaction vessel plays an important role in determining the final mean particle volume and particle size distribution. Prior art apparatuses and processes usually employ a high speed agitator disposed in the reaction vessel to mix the reactants. In some cases, a dispersing mechanism of the type shown in U.S. Pat. No. 3,415,650 (Frame & Johnson) is used to effect rapid dispersion of the reactants and to control the halide or silver ion concentration in the reaction vessel. U.S. Pat. No. 4,251,627 (Calamur) discloses an apparatus and a process for preparing a photographic silver halide emulsion having a controlled particle size, structure and size distribution; a silver salt and an alkali metal halide react to precipitate silver halide grains at uniform silver ion concentration (pAg). High velocity mixing jets are used, directed vertically downwardly to accomplish rapid dilution and mixing of reactants in a paddle-agitated reaction vessel. The arrangement there shown employs either a single recycled or a double recycled loop to provide additional dilution control and to feed a mixing jet.
In U.S. Pat. No. 3,917,485 (Morgan) a process for making internally sensitive photographic emulsion is disclosed. This process is applicable to convert a variety of surface sensitive or surface fogged silver halide emulsions including poly-dispersed emulsions of varied particle size into internally sensitive photographic silver halide emulsions by, for example, adding additional quantities of the silver and halide salts alternately so that the excess of ion above the stoichiometrically required amount is successively shifted between the silver and halide ions present.
None of the above-described apparatuses or processes is believed able to provide the necessary control over the dilution within the reaction vessel of the silver salt and the halide salt solutions especially when highly concentrated reactant streams are used. In addition, none appears to provide control over the proper mixing of these reactants, especially at the initial mixing point. Such control is believed to be desirable for controlled precipitation and growth of silver halide crystals. It is therefore believed advantageous to provide a process for the double jet precipitation of silver halide particles which includes accurate control of the dilution and mixing of the reagents. Such accurate control should eliminate fluctuations and run-to-run mixing variations which affect the silver halide particle size, and should thereby increase yields and reduce the need for blending of various batches to obtain the necessary particle size distribution. This is especially true when it is desired to operate at low silver ion concentration (high pAg) conditions in the precipitation vessel where reseeding is a problem or where high precipitation rates are desired. As a corollary, it is believed advantageous to provide a process for the production of silver halide crystals which reduces the need to use ammonia as a growth regulator. More particularly, in cases where ammonia has heretofore been used to produce larger grains than obtained without ammonia, it would be desirable to eliminate the ammonia, since it is hard to control the growth due to the activity of the ammonia. Additionally, the incorporation of dopants during double jet precipitation is related to the growth rate of individual crystals. Therefore, a process which allows the more uniform growth rate of individual crystals would allow the more uniform incorporation of dopants.